The present invention relates to a semiconductor integrated circuit which is suitable for high integration and, more particularly, to a semiconductor integrated circuit in which a potential having an absolute value smaller than a power potential to be supplied from an external power supply is used as a power potential to drive an internal active element circuit.
Semiconductor integrated circuits consisting of MOS transistors have been developed remarkably, and in the latter half of the 1960's, a semiconductor integrated circuit had been created in which tens or hundreds of MOS transistors, each having an effective channel length of about 10 .mu.m, were formed on one chip. Furthermore, the finer processing and higher integration of elements have advanced, so that recently a very large scale integrated circuit (VLSI) has been realized which includes a few hundred thousand of elements each having an effective channel length of about 1.5 .mu.m. In the future, it is expected that a semiconductor integrated circuit using submicron MOS transistors, each having an effective channel length of 1 .mu.m or less, can be formed.
In order to drive the MOS transistor whose effective channel length has been shortened in this way, it is necessary to use a driving voltage which is lower than a power supply voltage. This is because if this kind of MOS transistor is driven by the power supply voltage, a high electric field will be produced inside the MOS transistor causing various problems. From a viewpoint of the system's applications, it is desirable to commonly use a power supply for each integrated circuit constituting the system in consideration of the miniaturization and low cost. Furthermore, to increase the applicability to a system using a TTL, it is also preferable to set the power supply potential at 5 V, which is now used as the standard power supply voltage.
FIG. 1 shows a semiconductor integrated circuit comprising a function circuit 10 having MOS transistors which have short channel lengths, and an operating voltage supplying circuit 20 for supplying the operating voltage which makes this function circuit 10 operative.
This operating voltage supplying circuit 20 is formed of an MOS transistor 22. The drain of the MOS transistor 22 is connected to the voltage line VL, the source is grounded through a capacitor 23 and the gate is connected to a junction between the MOS transistors 21-P and 21-Q which are a part of the series of N MOS transistors 21-1, . . . , 21-P, 21-Q, . . . , and 21-N which are connected in series between a voltage line VL to which a voltage equal to a power supply voltage VC1 is supplied and the ground. The gate of each of the MOS transistors 21-1 to 21-N is coupled to its drain. In addition, the function circuit 10 is coupled between the source of the MOS transistor 22 and the ground. This function circuit 10 is, for example, a memory circuit including MOS transistors or the like which is needed as it is driven by a lower voltage than the power supply voltage VC1. All of the MOS transistors 21-1 to 21-N and 22 are of the enhancement type.
A reference voltage VR is applied to the gate of the MOS transistor 22. This reference voltage VR is obtained by dividing the power supply voltage VC1 in accordance with a ratio between a reciprocal of the conductance of the MOS transistors 21-1 to 21-P and a reciprocal of the conductance of the MOS transistors 21-Q to 21-N. Therefore, assuming that a threshold voltage of the MOS transistor 22 is VT, an operating voltage VC2 to the function circuit 10 is given by the following expression: EQU VC2=VR-VT (1)
In the above equation (1), the MOS transistor 22 is operating with the pentode characteristics and is set into the state close to the off state. When a current to be consumed by the function circuit 10 increases and the operating voltage VC2 becomes lower than (VR-VT), the MOS transistor 22 is rendered conductive, so that a large current is supplied to function circuit 10 from the voltage line VL through this MOS transistor 22, thereby compensating for the decrease in the operating voltage VC2. In the case where the consumption current of this function circuit 10 is relatively large even for a short time, since the current supplying ability of the MOS transistor 22 is not large enough, the consumption current is also supplied from the capacitor 23 as a discharge current to smooth the operating voltage VC2, thus preventing a remarkable reduction of this operating voltage VC2. In this way, to use this capacitor 23 to prevent the instantaneous reduction of the operating voltage VC2, this capacitor 23 must have large capacitance. However, to form a capacitor having a large capacitance in an integrated circuit, a large occupied area is required, which becomes an obstacle to the miniaturization and high integration of the circuit.
For example, in a dynamic RAM (d-RAM) of 64 kbits in which the voltages VC1 and VC2 are set at 5 V and 3 V, respectively, a peak current of about 150 mA will be consumed for an interval of about 15 nsec in the active operation mode. In this case, the capacitance C of the capacitor 23 which is needed to suppress the instantaneous reduction in voltage VC2 to be 10% or less is given by the following expression: ##EQU1##
If this capacitor of 7500 pF is formed by an MOS capacitor having an oxide insulator with a film thickness of 250 .ANG., the occupied area A needed for this capacitor is given by the following expression: ##EQU2##
This occupied area A is substantially equal to an area of a square whose side length is 2.33 mm. Since a chip area of an RAM having 2 .mu.m design rule is about 18 mm.sup.2 as well, the area of this capacitor will have to be at least about 30% of the chip area. Furthermore, it is necessary to suppress the variation in the operating voltage VC2 to be smaller than 10% in order to improve the operating margin of the function circuit 10, which results in the increase of the occupied area of the capacitor.
In this way, in the integrated circuit shown in FIG. 1, the occupied area of the capacitor 23 to suppress the variation in the operating voltage VC2 becomes so large that there is the drawback such that the degree of integration will decrease.